This invention relates to optical fiber acoustical sensors for use in detection and measurement of acoustic pressures. At the present time there are a number of electrical-mechanical microphones and hydrophones, such as piezoelectric, magnetostrictive, and capacitive transducers. These are used to detect and measure sound pressure and produce an electrical signal output. Accurate measurement of sound pressures is a problem as well as system design and impedance matching.
Presently certain fiber optic cabling has been subjected to stress to modulate coherent laser light as in U.S. Pat. No. 3,625,589 to K. Snitzer. This patent considers a number of arrangements for producing detectible phase shifts of coherent optical radiation propagated through optical fibers. In all cases Snitzer considers some type of mechanical transducer is activated by a signal that is to be measured (see Column 4, lines 37-40, "means for producing a signal, and control means responsive to said signal for applying mechanical stress to at least a section of said optical fiber," etc.). It is further noted that this has to be coherent light.
In other developments, a fiber optic communication channel may have its sheathing removed to expose a fiber to an acoustic source so that acoustic energy may be controllably coupled into the fiber optic path changing the refractive index so that there is a scattering of light energy traversing the fiber optic path. A portion of the optical radiation will emerge from the fiber optic path creating a terminal. Additionally, the fiber optic path may be modulated as desired to include signal information impressed from acoustic energy source and detected at the receiving end. An example of the acoustic energy source would be an electrical acoustic transducer.
It is, of course, known that fiber optic acoustic sensor elements consisting of coil optical fibers will be affected by the variations of sound pressure and changes in fiber diameter will result. This causes optical phase shift, Doppler frequency shifts, mode conversions and other changes in the propagation characteristics of the optical beam traveling through the fiber.
Other related developments include U.S. Pat. No. 2,972,051 which is an optical fiber radiation dosimeter. A bibliography of related articles and patents are attached.